Particularly in the area of direct-injection internal combustion engines that work by the diesel or four-stroke cycle method, the demands on injection systems with respect to the measured-out amount, the time, and the pattern of the injection are increasing continuously. Thus, injection patterns have been modified in recent years to the effect that either the injection amount to be measured out for a combustion cycle is divided into several small partial injections, or the rate pattern shaping is controlled by modulating the fuel pressure or by other rate-modulating measures. Appropriate measuring devices must be made available for this purpose.
Accordingly, a method is described in DE 31 39 831 A1 in which a measuring piston is moved by means of the fuel amount injected into a measuring chamber. The injection amount is decided from the path that the piston has traveled. After a certain number of individual injections, the measuring piston is conducted back into its starting position. The end positions of the measuring piston are recorded thereby in each case. However, due to the inertia of the piston mass and the occurring friction, measuring inaccuracies occur in this method that are too great for present-day conditions.
Thus, in DE 39 16 419 A1 an electromagnetically controlled measuring device is described that further develops the device according to DE 31 39 831 A1, whereby here the measuring chamber is emptied after each injection. While it is true that more accurate measuring results can be achieved by this due to the smaller total volumes to be measured, the problem of the relatively large piston mass to be moved still remains, so that vibrations and delays occur in the system as before. Thus, accurate measuring for example of a post-injection and its pattern analysis is not possible. Moreover the discharge of the respective measuring piston takes place with a discharge valve, so that injections following in very quick succession, as are usual in the normal operation of a combustion engine, cannot be resolved.
A further development of this device for improving the measuring accuracy is suggested in DE 44 34 597 A1. The return speed of the measuring piston is held constant, so that the switch delay of the magnetic valve can also be taken into consideration during emptying. In spite of this improved reproducibility of the starting position of the piston, the susceptibility of the system to vibration remains disadvantageous, not least due to the relatively large piston mass. Moreover, a resolution of closely-spaced individual injections and their patterns remains impossible due to the inertia of the total system.
In DE 41 30 394 A1, a measuring instrument for injection amounts is suggested, in which the injection takes place in a closed pressure container. After the incoming pressure in this pressure container has been measured after either the pre-injection or main injection, a valve is again switched so that the injected amount is drained into a measuring area in which a piston is again situated that is moved by the fluid, so that conclusions can be drawn about the injected volume from the movement of the piston. By means of such an embodiment it is indeed possible to resolve the injection processes with respect to the main or pre- or post-injection, but a quantitative statement about the injection patterns during for example a main injection remains impossible, since no continuous measurement of amounts is present. Moreover, such a design is not suitable for carrying out measurements on the engine while it is running, since several successive work cycles cannot be measured at that speed.
In WO 00/79125, the measuring principle is taken from DE 41 30 394 A1. However, the pressure in the pressure container is measured continuously, so that statements about the pattern of each individual injection are possible. The design is very complex, however, so that a large number of influencing variables reduce the measuring accuracy and reliability of the plant. Also, it is not possible to use such a device on an engine while it is running.
Alternatively, it is suggested in WO 02/054038 to make the movable piston lighter in order to avoid any post-vibration as much as possible, and to use a remote sensor to record the path to be measured, which sensor is capacitive or works on the eddy current principle. Through these measures the measuring accuracy is to be further increased.
In DE 1 798 080 an electronically controlled flow measuring and dosing instrument is again described that can measure flows with high accuracy over a large range. Due to its extremely low inertia, this measuring instrument is optimally suited for the immediate measurement of flows, but is not capable of reading synchronous-cycle flow rate information. This means that it is not possible to show the exact patterns of the injection processes to be measured and their periodicity simultaneously with the work cycle of a four-stroke or diesel engine.
With the exception of this device, it is common to all the known devices that only discontinuous flows can be measured downstream of the injection devices. This has the disadvantage that a flow rate measurement in combination with an optical analysis of the spread of an injection jet is not possible. Moreover, measuring accuracy is further limited.
A continuously working flow measuring instrument, which is installed downstream of the injection devices, is also disclosed by DE 33 02 059. The injection nozzle injects thereby into a conduit that leads to a gear pump and to which a second conduit is switched in parallel, in which second conduit slides a piston. These two conduits together form the necessary injection volume that can be changed by the movement of the piston. The path of the piston is measured and is also fed via an electrical closed-loop control circuit to a motor to control the speed of the gear pump. Due to the inertia of the piston and the fact that the physical properties of the injected fluid are not taken into consideration, the measurement of injected amounts with such a device gives insufficiently accurate measuring results.